1. Field of the Invention
The present invention relates to a method and a device for generating a two-phase gas-particle jet for treating surfaces by means of particles, in particular CO2 dry ice particles
2. Description of the Related Art
It is known that it is possible to clean surfaces by means of a compressed-gas jet, in particular compressed air, to which particles, for example of CO2 dry ice, have been admixed. The explanations given below relate to the use of dry ice particles, but can equally well be transferred correspondingly o other particles. The cleaning action is effected by She abrasive action of the particles and, in the case of dry ice particles, also by the cooling action of the CO2 dry ice particles which have been accelerated by the compressed-gas stream. On impacting on the surface to be cleaned, these dry ice particles transmit kinetic energy, and on this impact they break up into smaller fragments and sublime either on this impact or immediately afterwards, extracting heat from the surface, in addition to the cold-gas/particle mixture stream. The blasting agent, that is to say the CO2 dry ice particles, sublimes without leaving a residue. At most, loose particles from the former surface layer or surface contaminants remain on the surface to be cleaned, and these particles are deep-cooled and brittle, and can therefore be removed easily. In general, the surfaces are cleaned in such a manner that the surface particles removed are blown completely away from the surface during the blasting operation and are then collected by mechanical or pneumatic means.
It is known to generate the two-phase stream of compressed gas and solid CO2 dry ice particles by means of two fundamentally different methods:
In a first method, the CO2 dry ice particles are admixed with the compressed gas by means of an ejector, which Is known for example from U.S. Pat. No. 4,707,951, or a star feeder, and are then fed to a movable blasting nozzle via a common hose line. The ejector is designed in such a manner that the pressure nozzle ends with a minimum diameter in the axial region of the inlet funnel for the CO2 dry ice particles. The ejector method has the drawback that it is only possible to achieve relatively low particle velocities at the blasting nozzle, a fact which represents a severe limitation to the cleaning performance. Although the star-feeder method generates considerably higher particle velocities, owing to the possibility of setting higher gas pressures in the two-phase mixture, it has the drawback that firstly sealing problems on the star feeder may lead to disruption and, secondly, the action of the compressed gas means that sublimation losses inside the transport hose and into the blasting nozzle are high. These drawbacks impair the reliability and performance of the star-feeder method and increase process costs.
In a second method, compressed gas and CO2 dry ice particles are fed to a blasting gun with a directly connected blasting nozzle using the so-called two-hose method, i.e. via two separate hose lines. The blasting gun which is known, for example, from DE-195 44 906 A1 or U.S. Pat. No. 5,520,572 is in this case configured in the form of an ejector in such a manner that the compressed gas is guided through a high-pressure nozzle arranged axially with respect to the blasting nozzle, with the result that a reduced pressure is generated inside the blasting gun. In this case, a feed line for the CO2 dry ice particles is arranged radially and at an angle to the blasting nozzle, through which line these CO2 dry ice particles are sucked in and admixed to the gas jet, owing to the reduced pressure which is generated, it being necessary for the blasting nozzle, which is arranged directly on the blasting gun, to have a defined minimum length, so that the CO2 dry ice particles can be accelerated to a sufficiently high particle velocity.
The object of the invention consists in designing the surface treatment, in particular the cleaning, by means of particles, in particular CO2 dry ice particles, to be more efficient, i.e. to develop a method for generating a two-phase gas-particle jet and a device for treating surfaces using the two-phase gas-particle jet, which in particular increase the surface performance when treating surfaces by means of CO2 dry ice particles, make the cleaning process unsusceptible to problems and improve its technological reproducibility.
This object is achieved by means of a method for generating a two-phase gas-particle jet for treating surfaces by means of particles, in particular CO2 dry ice particles, in which the CO2 dry ice particles are fed with a tangential flow to a blasting chamber having an axis of flow, in such a manner that the CO2 dry ice particles are forced into a rotational movement about the axis of flow, and in which the angular velocity of this rotational movement is then increased in the direction of flow by means of a blasting nozzle.
The method according to the invention is distinguished by the fact that a pure compressed-gas stream and a second stream which contains CO2 dry ice particles are each fed to the blasting chamber separately via at least one compressed-gas feed line and via at least one particle-stream feed line, respectively, and are combined in the said blasting chamber in such a manner that the two-phase gas-particle jet is produced.
The abovementioned object is thus preferably achieved using the two-hose method described at the outset, in which a pure compressed-gas stream and a stream containing CO2 dry ice particles are fed to a blasting chamber in respectively separate feed lines and are combined therein, so that a two-phase gas-particle jet with an axis of flow is formed, the CO2dry ice particles being fed to the blasting chamber with a tangential flow in such a manner that the CO2dry ice particles are forced into a rotational movement about the blasting axis and that the angular velocity of this rotational movement is then increased in the direction of flow by means of a blasting nozzle.
Furthermore, the method according to the invention is configured in such a way that the rate at which the CO2 dry ice particles flow into the blasting chamber is configured to a maximum, by making the stream which contains CO2 dry ice particles a rapid compressed carrier-gas stream in at least one particle-stream feed line from a particle reservoir to the blasting chamber, and by the fact that the compressed carrier-gas component contributes, with a rotational movement in the same direction, to the formation of the two-phase gas-particle jet.
In a preferred form, the device according to the invention for treating surfaces by means of particles, in particular CO2 dry ice particles, using a two-phase gas-particle jet, has at least one turbostub for the supply of gas and/or particles, which is arranged on the housing of the blasting chamber and leads tangentially into the blasting chamber and has an additional axial alignment in the direction of the outlet of the boasting nozzle, the blasting nozzle being provided with an essentially conical inlet, the inlet angle of which is in total less than 120xc2x0, in particular less than 90xc2x0, preferably approximately 60xc2x0.
Advantageous configurations and refinements are given in the dependent claims. Accordingly, in an advantageous configuration the device is designed in such a manner that the blasting chamber is of cylindrical design in the region of the entry of the turbostub, the axial length of the blasting chamber corresponding to at least the diameter of the turbostub, preferably at least three times its diameter, and the internal diameter of the blasting chamber corresponding to at least 1.5 times the diameter of the turbostub, in particular approximately twice its diameter.
In particularly advantageous configurations of the device according to the invention, the compressed-gas feed line and the particle-stream feed line are produced parallel to one another from solid material over a length of 0.3 to 3 m, preferably approximately 1.5 m, with the axes of the feed lines being made either straight or bent.
Furthermore, the device is advantageously configured in such a way that the reservoir for the CO2 dry ice particles is connected to a ultrasonic transport ejector, the inlet funnel housing of which is connected to a compressed carrier-gas feed line for compressed carrier-gas which is at a relatively high pressure, and to an outlet stub connected by means of a hose to the blasting chamber, and has approximately the same nominal width, in which case the compressed carrier-gas feed line is connected to a convergent/divergent compressed carrier-gas ultrasonic nozzle, the outlet of which ends at the wall of an end chamber at the end of the inlet funnel housing, the internal diameter of the end chamber preferably corresponding to 1 to 3 times the nominal width of the outlet stub.
The advantages of the invention consist in a considerable increase in the surface performance when cleaning surfaces by means of CO2 dry ice particles, in the operating procedure being stabilized and in better reproducibility. Moreover, it has been found that the device according to the invention surprisingly makes it possible to use in a reliable manner dry ice particles which have a very large diameter, even of greater than 4 mm, with the result that new applications, in particular for the removal of relatively thick surface layers, can be realised. The solution according to the invention reduces the costs of surface treatment considerably and, if it is incorporated in blasting guns, reduces the physical strain on the operator when handling such devices.